A known variable valve timing control device is disclosed in Japanese Patent Laid-Open Publication No. 2001-3716. This known variable valve timing control device includes a first rotational body rotatably provided on a camshaft actuated being synchronized with the engine revolution and having a case provided with plural shoes on an internal peripheral surface, a second rotational body fixedly connected to the camshaft for slidably contacting an internal surface of the first rotational body and having a lock groove in axial direction on the sliding surface side, a lock member for locking the first rotational body and the second rotational body for unitary rotation by engaging with the lock groove and for releasing the lock by retracting from the lock groove, a biasing means for biasing the lock member towards the lock groove, and a hydraulic pressure supply means for applying the hydraulic pressure in the direction against the biasing force of the biasing means to the lock member. This variable valve timing control device further includes an engagement groove provided on at least one of the shoes in axial direction for retracting and supporting the lock member. At least one axial end of the groove is open. The side of the engagement groove facing the sliding surface of the second rotational body is open. The variable valve timing control device still further includes a plate shape lock member fitted in the engagement groove for sliding in the direction of the rotational center of the first and second rotational body, and a lock groove provided on a portion of a boss portion external peripheral surface of the second rotational body to be engaged with a tip portion of the plate shape lock member.
With respect to the known variable valve timing control device of the Japanese Patent Laid-Open Publication No. 2001-3716, the engagement groove is penetrated into at least one of the shoes of the case along the axial direction.
In the known variable valve timing control device of the Japanese Patent Laid-Open Publication No. 2001-3716, the first rotational body and the second rotational body are synchronized with each other to be rotated by the engagement of the lock member of the lock mechanism with a lock hole. In general, a cam provided on the camshaft of the internal combustion pushes down a valve body against the biasing force of the biasing means for biasing either an intake valve or an exhaust valve of the internal combustion engine (i.e., hereinafter referred as valve body) in closing direction. That is, the resistance applied to the cam when opening the valve body is large, and the resistance applied to the cam when closing the cam is small. Thus, the rotational speed of the camshaft is fluctuated relative to the rotational speed of the first rotational body (e.g., a timing pulley transmitted with the rotational force from a crankshaft via a belt) which rotates being synchronized with the engine revolution. More practically, the rotational speed of the camshaft is declined relative to the rotational speed of the first rotational body when the valve body is opened (i.e., when the cam is at a predetermined first phase). The rotational speed of the camshaft is increased relative to the rotational speed of the pulley when the valve body is closed (i.e., when the cam is at a predetermined second phase). By the change of the rotational speed in accordance with the rotational phase of the camshaft, the force for advancing or delaying the second rotational body (e.g., a rotor having a vane) relative to the rotation of the first rotational body is applied. The force applied to the second rotational body is also applied to the first rotational body via a lock portion for engaging the first rotational body and the second rotational body to be unitary rotated. In the known variable valve timing control device of the Japanese Patent Laid-Open Publication No. 2001-3716, the engagement groove engaged with the lock member is formed on one of the shoes formed on the case. The stress is repeatedly applied to the shoe portion including the engagement groove by the force for delaying and advancing the second rotational body relative to the first rotational body. Thus, it is required to ensure the strength of the portion around the engagement groove provided on the first rotational body. Notwithstanding, when reducing the size of the variable valve timing control device while managing to ensure the operation angle, it is difficult to ensure the strength of the shoe portion because the size of the shoe portion, particularly, the circumferential length is limited. In particular, provided that the engagement groove is provided on at least one of the shoes of the case along the axial direction, the shoe portion provided with the engagement groove has a structure like a cantilever, which may drastically decline the strength of the shoe portion.
A need thus exists for a variable valve timing control device which prevents the concentration of the load to a particular shoe portion for ensuring the strength thereof.